1. Field of the Invention:
The present invention relates to a process for the manufacture of electrodes in an integrated circuit, and more particularly to the formation of such electrodes in a charge coupled device.
The invention makes it possible to obtain electrodes or gates that are separated by the smallest feasible distance.
2. Description of the Prior Art:
With charge coupled devices (CCDs), it is common practice to define the electrodes in polycrystalline silicon (polysilicon) layers. After one such layer has been deposited on an insulating substrate, an etching operation is carried out through a pattern mask defining the electrode configuration. A constant problem in implementing this technique is to obtain a configuration having the required dimensions. In the present state of the art, it can be considered that while the dimensions are respected in the photolithography defining the pattern mask, there occurs a lateral retraction during etching. The extent of this retraction varies with the etching method used. In this respect, a distinction can be drawn between isotrophic etching, in which the removal of silicon occurs in the same manner in all directions, and anisotropic etching, in which it is possible to confer directional properties to the etching away of silicon by enhancing the action in the depth of the layer. But whichever the method used, lateral retraction remains inevitable. This problem is compounded by the fact that present circuits call for a growing number of electrode levels, leading to increasingly complex mask patterns for the different layers, unavoidable increases in etching periods, and consequently to a certain amount of over-etching of some layers, with relatively large shrinkages. To a certain extent, it is possible to compensate in advance for an over-etching by playing on the mask pattern. However, this kind of compensation is limited by the resolution of the photolithographic process and ceases to be applicable when dealing with very tight configurations, especially those of a periodic nature.
The present invention solves this problem and provides for very closely configured electrodes, in particular where inter-electrode spacings can be smaller than that obtainable with photolithographic resolution.